Synthetic aperture radar (SAR) is generally a system for forming an image or map of a terrain from a moving platform such as an aircraft, in which the return signals are processed to generate a set of relatively narrow synthetic beams in relation to the radiated beam.
More specifically, and referring to FIGS. 1A, 1B, 1C and 1D, there is disclosed a typical geometrical implementation of a synthetic aperture radar. The radar platform generally consists of a vehicle such as an aircraft 10 which carries the electronic equipment for radiating a scanning or fixed beam 12 resulting from the transmission of a radar pulse 14 and receiving the reflected radar return signal therefrom. The area of incidence of the transmitted radar pulse 14 comprises a swath in range .DELTA.R.sub.g and aximuth .DELTA..sub.az with .DELTA.R.sub.g having a minimum range R.sub.min and a maximum range of R.sub.max directed along the azimuth boresight R.sub.O. The geometry disclosed in FIGS. 1A and 1B is illustrative of a side look radar, for example wherein the radar beam is directed at a predetermined angle .alpha. relative to the direction of flight of the vehicle 10.
The significance of a synthetic aperture radar system is its ability to provide a set of effectively narrow subbeams within the illuminating scanning beam, which subbeams are defined as the synthetic beams, as illustrated by the reference numeral 17 in FIG. 1B. The synthetic beam is normally accomplished by including a reference frequency generator in the radar receiver which defines the axes of the synthetic antenna. An inertial navigation system (INS) provides a time history of the relative movement of the antenna phase center in some convenient ground coordinate system which is accurate to a small fraction of a radar wavelength for relative short periods of time, for example, the coherent integration time. The inertial navigation system provides information whereby motion compensation of the vehicle is provided. In U.S. Pat. No. 4,034,370, entitled "Second Order Motion Compensation for High Resolution Radar", issued on Jul. 5, 1977, which patent is incorporated herein by reference, there is described a method and apparatus for providing a digital implementation of dynamic motion compensation in azimuth, squinted, synthetic aperture radar by providing two stages of digital correlation utilizing discrete Fourier transform (DFT) or more preferably, fast Fourier transform (FFT) techniques. In such patent, there is described a first and second order of motion compensation which in effect causes the synthetic beams 17 to remain at fixed ground positions during the coherent integration time irrespective of aircraft maneuvers. In such system, following the first stage of correlation, the targets illuminated by the radar antenna pattern for each range gate are separated on a spectral basis in the time domain by an integrating digital filter bank, which in effect forms a plurality of synthetically generated subbeams. A unique second time translation and a phase shift is applied to each of the subbeams 17. Following such second time translation or second order motion compensation, a second and final DFT or FFT stage of correlation sequentially operates on each synthetic subbeam 17 which in effect divides each synthetic subbeam into a plurality of high resolution beam responses or azimuth cells at times referred to as synthetic sub-subbeams and illustrated at 18 in FIG. 1D representing the final motion compensated azimuth synthetic aperture high resolution data which is then applied to suitable SAR display output means.
Because of such fine resolution, or in other words, because of the relatively small ground area that each high resolution sub-subbeam response represents, the effect of range on the final output becomes paramount.
For example, as shown in FIG. 2A, an arrow 19 represents the direction of movement of the radar antenna, and the real beam 12, which is represented as conically shaped, and illuminates a particular ground area. Each of the beamwidths 12 illuminates ground areas that are separated approximately the distance travelled during one SAR coherent integration time T. Also shown on an instantaneous basis are the subbeams 17, some of which overlap one another toward the R.sub.max region noted by blobs 20. The subbeams 17 are magnified in FIGS. 2C and 2D, respectively, for R.sub.max and R.sub.min to illustrate the sub-subbeams 18, which constitute the final resolution of the SAR.
With appropriate motion compensation, the subbeams 17 and the sub-subbeams 18 remain fixed as the real beam 12 scans by. As apparent from FIG. 2A, block processing is rendered difficult. In that data collected during successive T seconds, gaps occur at and near R.sub.min and the overlaps toward R.sub.max cause processing of redundant data. This blocking problem is overcome with the teaching of the referenced U.S. Pat. No. 4,034,370, by utilizing two stage processing wherein the coherent integration time used to form the subbeams 17 is much shorter relative to the real beam 12 as shown in FIG. 2B so as to facilitate continuous processing. The sub-subbeams 18 (FIG. 1D) are assumed to be part of the subbeams 17 of FIGS. 1C and 1A, which have been omitted from FIGS. 1C and 1A for clarity purposes. In FIG. 2A, each of the blobs referred to at 20 represent the ground area corresponding to each of the subbeams 17 at the illustrated ranges, while blobs 21 represent the sub-subbeams 18 at maximum range R.sub.max, and each of the blobs 22 represent the ground area for each of such beams 18 at minimum range R.sub.min. Further, it should be noted that the ground area covered by the blobs 20 and 21 is much larger for each respective blob than the ground area of each of the blobs 20 and 22 at R.sub.min, thus tending to create distortion, particularly for wide range swaths.
Referring to FIG. 3, a three-dimensional diagram is shown to illustrate a non-rectilinear output of a map that exhibits a wide range swath prior to the present invention. Each of the contiguous blocks B in FIG. 3 represent an area segment that corresponds to a particular synthetic sub-subbeam 18 for a particular range gate. For example, the row of blocks B referred to at 23 represent terrain areas for a number of azimuth cells or subsubbeams 18 at the minimum range R.sub.min ; and the row of blocks B referred to at 24 represent the ground areas for such azimuth cells at maximum range R.sub.max. As apparent from FIG. 3, the actual terrain areas for the individual range cells of each azimuth cell increase in size from R.sub.min to R.sub.max. This resolution scale factor varies between the maximum range and the minimum range, and the variation of overlap of the synthetic beams (FIG. 2A) results in data block asynchronism and smearing of the output representation of the terrain when high resolution is desired. The ground areas corresponding to the individual synthetic sub-subbeams 18 in azimuth regardless of range should be practically identical and there should be data block synchronism, or in other words no overlapping area during each coherent integration time T, in order to provide the proper resolution for all range cells. Referring to FIG. 4, a three dimensional map is shown illustrating a desired rectilinear output of a map that is obtained when there is a constant azimuth scale factor and data block synchronism. The individual rows of data blocks C that correspond to the rows of data blocks B of FIG. 3 are similarly numbered.
Thus, it is desirable to provide a method and system for a high resolution synthetic aperture radar that provides data block synchronism and a constant azimuth scale factor over the range swath to provide continuous synchronized output during arbitrarily long mapping runs, and prevent smearing.